Proceedings of the Workshop
"The Magellanic Clouds and Other Dwarf Galaxies"
of the Bonn/Bochum-Graduiertenkolleg

First results of a CO survey of dwarfs

Marcus Albrecht, Roland Lemke, and Rolf Chini

Astronomical Institute, University of Bochum

Received 13th May 1998
Abstract. We have observed the J = 1-0 and J = 2-1 12CO emission towards the centers of 12 dwarf irregular galaxies using the 15 m SEST (Swedish ESO Submillimeter Telescope). The sources come from a complete FIR selected sample of 65 magellanic type galaxies. We were able to detect five of seven galaxies of type Sm, whereas we did not detect any signal from 5 Im types down to a 1 σ limit of 5-29 mK.

1. Introduction

The ratio of IR luminosity to gas mass LIR/Mgas is an ideal tracer to measure the activity and therefore the efficiency of star formation in galaxies. Typical values of LIR/Mgas are in the range around 5 for normal galaxies and around 100 for active galaxies. The IR luminosity, defined as the energy output between 12 and 1300 µm can be determined from IRAS and submm data while the measurement of the amount of ISM in a galaxy is a fundamental problem in extragalactic studies. There are two standard methods to determine the gas mass: via the CO luminosity or the 1300 µm dust emission. The conversion of CO luminosity into gas mass is given by the empirical relation
Eq. (1)
where the conversion factor β must be a function of the mean density and the temperature of the gas. The relation between the 1300 µm dust emission S1300 and the mass of the dust is given as follows:
Eq. (2)
where κν denotes the coefficient of absorption of the dust, Bν(Tdust) the Planck function and D the distance. With a gas-to-dust ratio R the amount of gas can then be determined by
Eq. (3)
Up to now the three important parameters β, κν and R have not been determined independently with the desired accuracy. Defining the luminosity at 1300 µm as L1300 := S1300·D2 the equations (1) and (2) yield the relation
Eq. (4)
which makes at least the product β·R·κ1300 a directly observable quantity; Tdust can be evaluated from submm continuum observations and from IRAS data.

2. Observations

Observations were made with the 15 m SEST (Swedish ESO Submillimeter Telescope) at La Silla/Chile in November 1997. The 15 m SEST has a half power beamwidth of 45" in the CO (1-0) line (115 GHz receiver) and of 24" for the CO (2-1) transition (230 GHz receiver). All observations were made in a double-beam-switching mode with a throw of 11' 45" in azimuth. Integration times varied between one and four hours per line. The sources come from a FIR selected sample of 65 dwarf galaxies (21 Im, 44 Sm) with IRAS flux densities f100 µm≥1000 mJy and optical diameters D25≤180" taken from Melisse and Israel (1994). We observed the central regions of 12 dwarf galaxies (5 Im, 7 Sm); positions were newly determined from the Digitized Sky Survey. The selection criteria for this particular observing run apart from visibility of the sources and sun avoidance of the telescope were small angular scale and high IRAS flux-density f100 µm.
Table 1. CO emission. Integration time tint is in hours, central velocity vLSR and line width dvLSR are in km s-1, main beam brightness temperature is in mK and ∫ Tmb dv is in K km s-1. In case of no detection the values for 1 σ rms are given in brackets
source/type transition tint vLSR dvLSR Tmb ∫ Tmb dv
NGC 899 CO(1-0) 3.7 - - (6.1) -
Im CO(2-1) 3.7 - - (10.7) -
NGC 1602 CO(1-0) 5.3 - - (5.8) -
Im CO(2-1) 5.3 - - (4.9) -
DDO 35 CO(1-0) 2.0 - - (12.1) -
Im CO(2-1) 2.0 - - (17.2) -
2 Zw 40 CO(1-0) 2.7 - - (9.0) -
Im CO(2-1) 2.7 - - (13.2) -
NGC 2915 CO(1-0) 1.7 - - (18.8) -
Im CO(2-1) 1.7 - - (29.4) -
NGC 145 CO(1-0) 1.67 4081 54 37.2 2.14
Sm CO(2-1) 1.67 4076 47 56.9 2.25
NGC 2730 CO(1-0) 3.67 3762 151 8.7 1.39
Sm CO(2-1) 3.67 3741 101 10.7 1.15
NGC 3246 CO(1-0) 1.34 2104 168 9.9 1.78
Sm CO(2-1) 1.34 2099 46 12.3 0.60
NGC 3659 CO(1-0) 3.0 1255 150 11.4 1.82
Sm CO(2-1) 3.0 1257 79 15.4 1.30
NGC 7107 CO(1-0) 4.0 2186 49 6.0 0.31
Sm CO(2-1) 4.0 2204 30 9.2 0.30
NGC 7162A CO(1-0) 1.3 - - (7.1) -
Sm CO(2-1) 1.3 - - (16.8) -
NGC 7732 CO(1-0) 2.0 - - (7.3) -
Sm CO(2-1) 2.0 - - (8.3) -

3. Results

We were able to detect CO in 5 galaxies of type Sm: NGC 145, NGC 2730, NGC 3246, NGC 3659 and NGC 7107. Table 1 shows the preliminary results of the line analysis by gaussian fits. Antenna temperatures have been converted to main-beam brightness temperatures Tmb assuming main-beam efficiencies ηmb=0.71 at 115 GHz and ηmb=0.51 at 230 GHz appropiate for point sources.

References

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First version: 04thAugust,1998
Last update: 04thOctober,1998
Jochen M. Braun   &   Tom Richtler
 (E-Mail: jbraun|richtler@astro.uni-bonn.de)